Emphasize `sender`/`psender` usage of `msg.sender` in README

[0xicingdeath]

When used in property mode with echidna_x, psender defines the address transactions originate from (from a single address).

When used in assertion mode, echidna uses sender, in which it originates from 3 addresses.

https://github.com/crytic/echidna/blob/v2.0.0-b2/tests/solidity/basic/default.yaml#L35-L36

This is not explicitly clear unless you look at the default.yaml.

[montyly]

Related to https://github.com/crytic/building-secure-contracts/issues/80